This invention relates to polymeric compositions having increased resistance to electrical treeing and water treeing properties, said compositions being useful as insulation for electrical cables.
Polymeric compositions are well-known and are used extensively as insulation materials for wire and cable. As an insulator, it is important that the composition have various physical and electrical properties, such as resistance to mechanical cut through, stress crack resistance and dielectric failure. Recent publications have indicated that water tree growth and electrical tree growth in the insulation are particularly important problems since they are associated with, though not necessarily totally responsible for, dielectric failure.
An important application for an insulation material is in high voltage transmission and distribution cable, especially in direct buried underground service and three types of trees have been observed in power cables, to wit, electrical trees, water trees and electrochemical trees. It is generally believed that electrical trees are generated by corona discharges causing fusion and breakdown of the polymer, whereas water trees are usually observed in cables buried in wet locations and have a different appearance compared to the electrical trees. The eletrochemical trees are similar to the water trees but are characterized by the presence of metal ions in the trees.
U.S. Pat. No. 4,144,202 granted to Ashcraft et al. relates to inhibiting the electrical breakdown of insulation by water treeing in dielectric materials based on ethylene polymers. This patent discusses electrical failures which are due to treeing and explains the concept of treeing and some of the causes for treeing. In general, as the polymeric composition breaks down the damage progresses through the insulator, or dielectric, in a path that looks something like a tree. Treeing usually is a slow type failure and may take years to cause a failure in the insulation. As disclosed in the patent, water treeing is inhibited in the ethylene polymer compositions by employing therein certain organo silane compounds. In particular, the organo silane is a silane containing an epoxy containing radical. Suitable polymers, adjuvants and processing procedures for preparing the composition are described in the patent, which patent is hereby incorporated by reference.
U.S. Pat. No. 4,206,260 granted to McMahon relates to insulation particularly suitable for high voltage power cable containing an effective amount of an alcohol of 6 to 24 carbon atoms which imparts electrical tree growth resistance to the composition. This patent, as in U.S. Pat. No. 4,144,202, supra, contains a discussion of the electrical treeing problem in polymer compositions and cites numerous patents attempting to overcome this problem. Suitable polymers, adjuvants and preparation procedures are noted therein and this patent is hereby incorporated by reference.
German Offenlegungsschrift No. 2,737,430 discloses that certain alkoxysilanes added to polyolefin insulation prevent water-tree formation. Several trimethoxy and triethoxy silanes are said to be useful. No alkoxyalkoxy silanes are taught or suggested as having both water treeing and electrical treeing inhibiting properties.
U.S. Pat. No. 3,553,348 granted to Betts, British Pat. No. 1,248,256 granted to General Electric Company and British Pat. No. 1,277,378 granted to General Electric Company relate to mineral filled polymer compositions useful as electrical wire and cable insulation. The mineral filler is treated with an organosilane such as an alkyl alkoxysilane or a vinyl alkoxysilane to decrease the porosity of the composition. None of these patents teach or suggest that addition of an organosilane to an unfilled polymer composition will beneficially enhance the water treeing and electrical treeing resistance of the polymer composition.
Unfortunately, however, the prior art has not provided an insulation composition having both increased resistance to water treeing and electrical treeing. As noted in U.S. Pat. No. 4,144,202, supra, intrinsic electric breakdown, failure by corona, electrical treeing and water treeing are different and the mechanisms for each are different and a different solution is required to effect an improvement in a dielectric material for each mode of failure involved. Thus, the problem of providing a single composition capable of resisting both electrical treeing and water treeing is a formidable one confronting the art.